The present invention generally relates to fuel cells for generation of electricity and, more particularly, to a liquid hetero-interface fuel cell device.
As the worldwide energy shortage worsens, fuel cells have become attractive because of their high efficiency, low emission characteristics and exceptional reliability. Conventional methods of converting chemical energy of hydrocarbon fuels into electricity involve combustion. Such methods may use various types of steam turbines or internal combustion engines whose thermodynamic efficiency is limited to about 40%, with 25% being an average efficiency. Galvanic cells provide an alternate approach to converting chemical energy into electricity.
A galvanic cell which can oxidize hydrogen or hydrocarbon fuel is known as a fuel cell. Not limited by the Carnot or Stirling cycle efficiencies, a fuel cell can achieve a thermodynamic efficiency over 50% and possibly even higher than 80%. Economically viable fuel cells would find applications ranging from propulsion of automobiles, trains, and aircraft to generation of electricity in utility power stations, industrial plants, or private homes. Wide deployment of fuel cells would permit doubling or tripling the extractable energy from existing fossil fuel resources, thereby alleviating the energy shortage. Unlike conventional combustion, fuel cells do not pollute the environment and reduce contributions to the xe2x80x9cgreen house effect.xe2x80x9d Of the numerous fuel cell concepts investigated in the last 90 years with varying degree of success, however, relatively few were advanced into commercial products.
Probably the most successful fuel cell developed in a variety of forms uses gaseous hydrogen fuel and oxygen oxidizer. Some of these devices were developed into commercial products for use in space, or in remote applications such as marine buoys. Beyond these niche, high-value applications, however, fuel cells have not won wide acceptance. Reasons for this situation include low availability and poor storability of certain easy-to-use fuels (as is the case with hydrogen), difficulty in achieving complete oxidation of more complex hydrocarbon molecules, poor electric conductivity of commonly available petroleum-based fuels, and high cost of electrodes due to the use of rare elements or noble metals. One of the problems is that the membrane separating the fuel and the oxidizer must allow the chemical species to be physically transported across the membrane while the electrons associated with the oxidation-reduction reaction are collected and separately flow through the external load. A voltage is maintained across the membrane by the chemical potential gradients of the reacting species, which serve to separate electrical charge. Carbon compounds and intermediates do not readily cross the conventional membranes and so reaction stops. CO2 is also a gaseous compound and must be removed in some way. These problems are sufficiently serious that, in current hydrocarbon fuel cells, the hydrogen is simply stripped away in the reforming process and the energy associated with carbon oxidation rejected as waste heat. Besides being bulky, heavy, and hazardous to operate, reformers add complexity and reduce efficiency of the fuel cell system as the energy in the fuel attributable to its carbon content is largely wasted in most such designs.
Fuel cell designers must also overcome numerous challenges which restrict operating characteristics of the cell such as removal of reaction products (typically carbon dioxide and/or water), lifetime of electrodes, and poisoning of electrolyte by parasitic reactions. The ideal fuel cell would use widely available, easily storable, low cost fuels (e.g., kerosene, alcohol, natural gas) and atmospheric oxygen. Construction and operation of the cell should allow it to compete against established electric power generating technologies in specific market segments. Some of the considerations in designing a fuel cell are reactivity, invariance, oxidizers, catalysts, cell separators, and polar and non-polar fluids.
Reactivity relates to both the speed and completeness of the reaction. Reaction speed requires high electrode activity, which is controlled by the rates and mechanisms of electrode reactions, and results in high current densities. Reaction completeness requires proper stoichiometry. For example, carbon should always be oxidized to CO2 rather than CO so that a maximal amount of electrical energy is released in the reaction. In prior art, the reactivity requirement has been met by using porous materials to enlarge the active area of electrodes, by increasing pressure, by raising temperature, or by using catalysts.
Invariance relates to the objective that a fuel cell, unlike a conventional battery, should maintain constant performance throughout its life. This implies that there should be no corrosion or side reactions, and no changes in the electrolyte or the electrodes. In particular, fuel should not diffuse over and mix with the oxidizer. Catalysts can become poisoned and the pores of gas electrodes can become clogged with liquid (xe2x80x9cdrowningxe2x80x9d), gas (xe2x80x9cblowingxe2x80x9d), or extraneous material making the electrode inoperative. If xe2x80x9cwrongxe2x80x9d ions carry the current, the electrolyte may lose its invariance, and the cathode and anode reactions may be thrown out of balance.
Oxidizers relate to the fact that most fuel cells use oxygen for fuel oxidation. Oxygen is first cathodically reduced to OHxe2x88x92 cations, which react in the electrolyte with anions originating from fuel. Unfortunately, reactivity of OHxe2x88x92 with many fuels is very slow, which leads to impracticably low current densities. While catalysts can often remedy this situation, they typically require use of expensive materials such as platinum or palladium, hence driving up the capital cost of the fuel cell system. Reactivity can be also increased by choosing a more reactive oxidizer such as the O2Hxe2x88x92 cation.
Catalysts previously used with fuel cells are typically in the form of coatings on electrode surfaces. Recently, a new soluble catalyst has been introduced, which is suitable for increasing reactivity of H2O2 in oxidizing a broad variety of organic substances. This soluble catalyst is methyltrioxorhenium (CH3ReO3), also known as methylrhenium trioxide or MTO. Synthesis of MTO was first reported in 1979 and its use as a catalyst for hydrogen peroxide oxidation of a number of alkenes, alkynes, and ketones was first published in 1991 by W. A. Hermann et al. in the journal Angew. Chem., Intl. Ed. Eng., vol. 30, pp. 1638-41. This catalyst has important attractive features including ease of synthesis, stability in the air, stability and solubility in aqueous (low pH) as well as organic solvents, low toxicity, and effectiveness as either a homogeneous or heterogeneous catalyst. Unlike other catalysts, MTO alone does not decompose H2O2. Research shows that addition of a cocatalyst (preferably bromine ions) can further accelerate processes catalyzed by MTO as published in 1999 in the article xe2x80x9cBromide ions and methyltrioxorhenium as co-catalysts for hydrogen peroxide oxidations and brominations,xe2x80x9d by J. H. Espenson et al. in the JournalOrg. Chem., vol. 54, pp. 1191-96.
Cell separators relate to the fact that it is impractical to mix large volumes of fuel and oxidizer. In most fuel cells, fuel and oxidizer are maintained in different compartments of the cell sharing a common wall known as a separator. Such a separator is permeable so that the fuel or the oxidizer can be contacted and reacted in a controlled fashion. Oxidation of the fuel takes place on the surface or within the separator. To promote high reaction rates, fuel cell separators often contain catalysts. A variety of separator designs have been used with varying degrees of success, including porous beds and ion exchange membranes. Key issues in design of fuel cell separators include maintaining high transport rates for reacting species and reaction products, and low susceptibility to flooding.
Polar and non-polar fluids relate to the well known fact that oil and vinegar do not mix. This is because the molecules of oil are non-polar, i.e., they have no net electrical dipole moment (product of charge times separation), whereas the molecules in the aqueous solution of acetic acid (vinegar) do have a net dipole moment. The result is that polar molecules attract each other strongly and tend to exclude non-polar molecules, thus forming separate regions separated by a boundary called a meniscus. There is surface free energy associated with the boundary that is manifested as surface tension. On an atomic scale, the boundary is indistinct with a gradual transition of composition from polar to non-polar molecules over a very short distance. The concentration gradients can be described as chemical potential gradients in a way analogous to electrical potential gradients. This heterointerface between polar and non-polar liquids is very similar in many respects to the Pxe2x80x94N homojunctions and heterojunctions familiar from the field of semiconductor devices, where holes and electrons are the species maintained in separate regions of a continuous solid.
The first galvanic cell converting hydrocarbon fuels such as petroleum, stearic acid, and starch into electricity was demonstrated in 1910 by I. Taitelsbaum published in Z. Elektrochem., vol. 16, p. 295. Cells working with a number of other gas or liquid hydrocarbon fuels were successfully demonstrated in the following years, most notably after the second world war. One attractive feature of a fuel cell is its simplicity. For example, a fuel cell can be as simple as two electrodes immersed into an electrolyte containing a mixture of alcohol and potassium hydroxide. A conventional configuration of a fuel cell 100 is shown in FIG. 1. As seen in FIG. 1, fuel cell 100 includes a container 102, porous electrodes comprising an anode 104 and cathode 106, and electrolyte 108. Fuel gas 110 enters container 102, diffuses through anode 104, and is oxidized, releasing electrons 112 to an external circuit connected to load 114, where useful work may be performed. Oxidizer 116 enters container 102, diffuses through cathode 106, and is reduced by electrons 112 that have come from anode 104 by way of external circuit connected to load 114. Oxidation products 118 may be produced and expelled as waste. Fuel cell 100 may also produce waste heat.
The literature is replete with material relating to numerous aspects of fuel cell technology, for example, see xe2x80x9cFuel Cell Systemsxe2x80x9d by R. F. Gould, ed., published in 1965 by the American Chemical Society, Washington, D.C. Probably the most successful fuel cell developed in a variety of forms uses gaseous hydrogen fuel and oxygen oxidizer. An example of one of these devices, known as H2xe2x80x94O2 fuel cells, is illustrated diagrammatically in FIG. 2. FIG. 2 shows fuel cell 200 including container 202 forming gas cavities 201 and 203, porous electrodes comprising anode 204 and cathode 206, and membrane 208. Hydrogen 210 enters container 202 into gas cavity 201, diffuses through anode 204, and is oxidized in the reaction:
H2+2OHxe2x88x92xe2x86x922H2O+2exe2x88x92
releasing electrons to an external circuit (not shown in FIG. 2) which may be connected to a load, where useful work may be performed. Oxygen 216 enters container 202 into gas cavity 203, diffuses through cathode 206, and is reduced in the reaction:
xc2xdO2+H2O+2exe2x88x92xe2x86x922OHxe2x88x92
by electrons that have come from anode 204 by way of the external circuit which may be connected to a load. The overall reaction is
H2+xc2xdO2xe2x86x92H2O
so that water is produced and which may be expelled as waste oxidation products and inerts 218 from fuel cell 200.
Some H2xe2x80x94O2 fuel cell devices were developed into commercial products for use in space, or in remote applications such as marine buoys. However, due to the low availability and poor storability of hydrogen, H2xe2x80x94O2 fuel cells have not won a wide acceptance beyond these niche, high-value applications.
Difficulty in achieving complete oxidation of more complex hydrocarbon molecules and poisoning of cathode electrolyte by CO2 has thus far prevented successful development of an efficient fuel cell working directly with hydrocarbon fuels. See, for example, xe2x80x9cHydrocarbon Fuel Cell Technologyxe2x80x9d by B. S. Baker, ed., published in 1965 by Academic Press, New York, N.Y. Owing to their poor conductivity, nonpolar hydrocarbon fuels have been particularly difficult to work with. Consequently, industrial or electrical utility applications are usually based on reforming natural gas or another hydrocarbon to produce hydrogen that is used in the cell to actually produce electricity. FIG. 3 shows several approaches to the reformer-fuel cell architecture developed in the prior art. FIG. 3 represents processes normally used in conjunction with fuel cells 100 and 200 in a more abstract graphical form for the sake of simplicity. For comparison, a direct oxidation process 310 is shown near the top of FIG. 3. The direct oxidation process 310 comprises processes of hydrocarbon fuel 301 diffusing through anode 302, and being oxidized, where CO2 may be expelled as waste oxidation product 305, releasing electrons to an external circuit (not shown) connected to a load where useful work may be performed. Oxidizer comprising air 303 diffuses through cathode 304, where oxygen is separated from nitrogen 307, nitrogen 307 is expelled as waste, and oxygen in air 303 is reduced by electrons that have come from anode 302 by way of the external circuit. The overall reaction producing electricity is facilitated by electrolyte 306. This process is most desirable as it allows direct oxidation of hydrocarbon fuel without a need for external reforming process and promises highest energy extraction from the fuel. However, in prior art, this process has been very difficult to implement, thereby proving a motivation for development of reforming processes described below.
Continuing with FIG. 3, external reformer process 320 may be similar to direct oxidation process 310, but further comprises processes of reforming the hydrocarbon fuel in a reformer 328 and purifying the reformed fuel in a purifier 329, as known in the art, before diffusing the fuel, now in the form of hydrogen 321 into anode 322. Also as seen in FIG. 3, internal reformer process 330 may be similar to external reformer process 320, but the reformer, purifier, and anode have been combined into a single unit 338, as known in the art, to achieve certain gains in efficiency. Finally, as seen in FIG. 3, partial oxidation molten carbonate process 340 comprises a process of partially oxidizing the hydrocarbon fuel in partial oxidizer 348 before diffusing the fuel into anode 342. Chemical reactions producing electricity are facilitated by molten carbonate 346, as known in the art. Carbon dioxide is usually a waste product in the reforming process and typically the heat of formation for the CO2 is rejected as waste heat and does not produce electric power in an external circuit.
As can be seen, there is a need for a fuel cell that uses widely available, easily storable, low cost fuels such as kerosene, alcohol, and natural gas and that uses atmospheric oxygen as oxidizer. There is also a need for a fuel cell the construction and operation of which allows it to compete against established electric power generating technologies in specific market segments. Furthermore, there is a need for a fuel cell which exhibits efficient removal of reaction products such as carbon dioxide and water, long and stable lifetime of electrodes, and reduced poisoning of electrolyte by parasitic reactions.
The present invention provides a fuel cell that uses widely available, easily storable, low cost fuels such as kerosene, alcohol, and natural gas and that uses atmospheric oxygen as oxidizer. The present invention also provides a fuel cell the construction and operation of which may allow it to compete against established electric power generating technologies in specific market segments. The present invention further provides a fuel cell which exhibits efficient removal of reaction products such as carbon dioxide and water, long and stable lifetime of electrodes, and reduced poisoning of electrolyte by parasitic reactions.
In one aspect of the present invention, a fuel cell device for generation of electricity from a polar oxidizer liquid and a non-polar fuel fluid includes a separator for separating the polar oxidizer liquid from the non-polar fuel fluid. The separator is made from a material that is hydrophobic with respect to the polar oxidizer liquid, and has a large number of small apertures, which are appropriately sized and spaced to provide a direct, controlled contact between the polar oxidizer liquid and the non-polar fuel fluid.
In another aspect of the present invention, a fuel cell device for generation of electricity from a conductive polar oxidizer liquid and a non-polar fuel fluid includes a cathode in contact with the polar oxidizer liquid; an anode in contact with the non-polar fuel fluid; and a separator for separating the polar oxidizer liquid from the non-polar fuel fluid. The separator is made from a material that is hydrophobic with respect to the polar oxidizer liquid, and has a plurality of apertures, which are appropriately sized and spaced to form a meniscus in each aperture. The meniscus provides a controlled contact surface and forms a liquid heterointerface between the conductive polar oxidizer liquid and the non-polar fuel fluid in and about which liquid heterointerface oxidation processes occur.
In still another aspect of the present invention, a fuel cell device for generation of electricity from a conductive polar oxidizer liquid and a non-polar fuel fluid includes a cathode in contact with the polar oxidizer liquid; an anode in contact with the non-polar fuel fluid; and a separator for separating the polar oxidizer liquid from the non-polar fuel fluid. The separator is made from a material that is hydrophobic with respect to the polar oxidizer liquid, and has a plurality of apertures, which are appropriately sized and spaced to form a meniscus in each aperture. The meniscus provides a controlled contact surface and forms a liquid heterointerface between the conductive polar oxidizer liquid and the non-polar fuel fluid in and about which liquid heterointerface oxidation processes occur. The fuel side of the separator is coated with a conductive material to form the anode, the conductive material is in electric contact with the perimeter of the meniscus, and the cathode is formed on the oxidizer side of the separator.
In a further aspect of the present invention, a method for generation of electricity from a conductive polar oxidizer liquid and a non-polar fuel fluid includes steps of placing a cathode in contact with the polar oxidizer liquid; placing an anode in contact with the non-polar fuel fluid; and separating the polar oxidizer liquid from the non-polar fuel fluid, using a separator made from a material hydrophobic with respect to the polar oxidizer liquid. The separator has a number of apertures, which are appropriately sized and spaced to form a meniscus in each aperture. The meniscus forms a liquid heterointerface between the conductive polar oxidizer liquid and the non-polar fuel fluid in and about which liquid heterointerface oxidation processes occur.